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Quantum Information and Measurement (QIM)

Repeated multi-qubit readout and feedback with a mixed-species trapped-ion register

Matteo Marinelli, Vlad Negnevitsky, Karan K. Mehta, Hsiang-Yu Lo, Christa Flühmann, Jonathan Home

Published Jan 1, 2019• 0 citations
Publisher

Abstract

Using a mixed-species ion chain, we demonstrate up to 50 sequential parity measurements between two beryllium ion qubits coupled to a co-trapped ancillary calcium ion. With feedback on the beryllium qubits conditioned on the ancilla readout we stabilize Bell states and parity subspaces. [1]
DOI
10.1364/qim.2019.s2d.5

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Abstract Fault-tolerant quantum computing relies on the ability to detect and correct errors, which in quantum error correction codes is typically achieved by projectively measuring multi-qubit parity operators and by conditioning operations on the observed error syndromes. Here, we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the ZZ and XX parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of F ≈ 74% in up to 12 cycles of the feedback loop. We also perform the protocol using Pauli frame updating and, in contrast to the case of real-time stabilization, observe a steady decrease in fidelity from cycle to cycle. The ability to stabilize parity over multiple feedback rounds with no further reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.
Scalable quantum networks require generating remote entanglement faster than decoherence erases it, calling for nodes that combine efficient light-matter interfaces with long coherence times. Cavity-coupled trapped ions are a promising platform for network nodes, offering high-efficiency extraction of photons. Here, we report multi-second coherence for a qubit register in a cavity-integrated ion trap. First, using dynamical decoupling on spin ground states, the coherence times of five co-trapped ion qubits are extended into the multi-second regime. Second, cavity-collected ion-photon entanglement is faithfully stored in these protected ion-memory states for multiple seconds. Third, we show that ion-memory qubits can be robust to the generation of over a thousand cavity photons by a co-trapped ion, with decoherence limited by laser crosstalk. Finally, we estimate that a modestly improved and duplicated ion-cavity system could enable the simultaneous establishment of multiple remote Bell pairs between two remote ion registers, representing a step toward networking multi-qubit prototype trapped-ion quantum processors.
Trapped ions technology is the most promising candidate for construction of the large-scale quantum computers, quantum repeaters and quantum networks. Quantum networks require matter qubits with fast quantum gates and measurement, efficient coupling to photons and low gate error rates. There was growing progress in two-species ion trapping [1], where one ion acts as a communication qubit that can be efficiently entangled to photon and the other one used as excellent quantum memories allowing efficient local processing. However, the photonic interface needed for communication ions must not affect the state of the quantum memory ion. In a recent experimental breakthrough, the isolation between quantum memory ion (171Yb+) and communication ion (138Ba+) was achieved and a quantum state transfer between the ions has been demonstrated.